1. Field of the Invention
This invention relates to an amplifier for analog signals in which the amplifier transfer function is controlled by a digital code, and more particularly to an amplifier in which a current transfer cell is controlled in response to a multiple bit digital control signal.
2. Description of the Prior Art
Prior attempts to control the amplification transfer function for an analog signal in response to a digital control signal have utilized discrete components. For example, when it is desired to use the output of a digital computer to control an analog signal, the analog signal has typically been fed through a voltage controlled amplifier, and the output of the computer has been processed through a digital-to-analog converter (DAC) to place it in an analog format suitable for controlling the voltage controlled amplifier. A popular type of amplifier used for this purpose is disclosed in U.S. Pat. No. 4,234,804, assigned to DBX Corporation.
The addition of a separate DAC tends to result in a mismatching between the computer and the amplifier, requiring retrimming of the converter and making it more difficult to achieve repeatability in volume production. Furthermore, combining such discrete devices makes the resultant digital controlled amplifiers quite temperature sensitive, with large temperature drifts in the order of 3000 parts per million per degree centigrade. The amplifier's control range is also quite limited, as is its linearity.